Flashback Pro allows you to select any available audio device for sound playback but, in most cases, you will need it to be set to the same device that Windows is using as its output device.
If you can see that there is sound but cannot hear it, you are probably trying to listen to the sound using a device that doesn't match the Output device for Windows.
If you go to Sound -> Sound Options… in the Player, you can switch the playback device that Flashback Pro uses.